The invention relates to the treatment of aluminum surfaces, and more particularly to the treatment of aluminum surfaces to provide a surface thereon suitable for use in the production of lithographic printing plates.
There are many methods and processes which have been heretofore employed in the treatment of aluminum surfaces to render them suitable for use in the production of lithographic printing plates. One such method involves the electrolytic treatment of aluminum, for example, electrolytic etching by use of a hydrochloric acid electrolyte. Various prior art publications, for example, U.S. Pat. Nos. 3,072,546 and 3,073,765 and British Pat. Nos. 879,768 and 896,563 describe the treatment of aluminum surfaces with hydrochloric acid while applying an alternating current to the aluminum plates to render the plates suitable for lithographic use.
In the treatment of such aluminum association alloys as 1100, a relatively large amount of electrical power has been required to obtain the degree of etching desired. It has also been found in the practice of the prior art processes that uniform etching of the surface is not obtained, and the character of the grain imparted to the surface is not consistent, portions thereof being relatively coarser than others, thus yielding an undesirable irregular surface which is not ideally suitable for lithographic use. When the surface of the aluminum sheet is irregular and non-uniform, it can interfere with the subsequent printing process when the surface is subsequently coated with a photosensitive resin as is employed in normal lithographic processes and is well known to the skilled worker.
Heretofore, various suggestions have been made to overcome the disadvantages encountered in the practice of the prior art processes. One such suggestion in U.S. Pat. No. 3,963,594 involves the use of a hydrochloric acid and gluconic acid electrolyte for etching. Other suggestions such as those contained in U.S. Pat. Nos. 3,342,711; 3,365,380 and 3,366,558 refer to an electrolytic polishing effect obtained on aluminum and other metals using a mixture which may include various electrolytes such as sulfuric acid and gluconic acid.
To date, all known processes for the electrochemical graining of aluminum have only been able to grain one side of the sheet. However, it is frequently desired to grain both sides of the sheet in a uniform manner. Past methods have required a costly double sequence graining treatment where one side of the sheet is grained first and then the other side is grained in a second step. Each one side graining step not only grains that full side but a portion of the edge of the reverse side. Thus when each side was heretofore singly grained, the edges of both sides were disadvantageously double grained causing a non-uniformity across the plate surfaces. All such methods employ single phase alternating current at line frequency (50 to 60 Hz) although an increased frequency is known to produce some benefits. The present invention very importantly provides surface uniformity from one side of the web to the other. This is most important when a lithographic printing plate is produced using a substrate made by the process of this invention because exposure of the light sensitive coating will be uniform from one side to the other with a predictable ink/water balance when printing. The present invention provides a method for uniformly graining both sides of an aluminum sheet substrate simultaneously employing three phase alternating current. Graining on each side of the web is noticed to be more uniform than two single phase graining operations under equivalent electrolyzing conditions. In addition, the graining is achieved at a substantial power savings over two sequential single phase one-sided graining treatments.